1. Field of the Invention
The present invention relates generally to a mobile phone and, more particularly, to a noise suppression circuit for a mobile phone.
2. Description of the Related Art
In a mobile phone, a noise suppression circuit is used on an audio line to reduce white noise that commonly exists during an ongoing call.
FIG. 1 illustrates a conventional noise suppression circuit for a mobile phone. Referring to FIG. 1, a Radio Frequency (RF) signal received by an antenna is passed through an RF unit 103 and then fed to a vocoder 105. The vocoder 105 extracts a digital audio data from the RF signal, and feeds the extracted digital audio data to a coder/decoder (codec) 107. The codec 107 converts the digital audio data into an analog audio signal and feeds the analog audio signal to a noise reduction filter 109, at which high frequency components of the analog audio signal are attenuated. The analog audio signal filtered through the noise reduction filter 109 is output through a speaker. The noise reduction filter 109 is a low-pass filter including a resistor and a capacitor.
In such a conventional noise suppression circuit, sound gain is increased or decreased depending upon presence or absence of an audio signal in order to reduce the white noise. However, the capacitance of the capacitor of the low-pass filter does not vary, regardless of the presence or absence of an audio signal. However, a high-capacitance capacitor provides good noise suppression but severe sound distortion, and a low-capacitance capacitor provides poor noise suppression.
Further, in the conventional noise suppression circuit, sound volume is reduced for noise suppression when an audio signal is absent, and sound volume is increased to a normal level when an audio signal is present. Thereby a software-processing burden of sound gain control is increased. Although reduction of sound gain is effective in suppression of noise in an initial audio signal, it may be ineffective in suppression of noise generating from the codec and subsequent devices, and of white noise, including popup noise.